1. Field of the Invention
The present invention relates to a light guide, an illuminating device having the light guide, and an image reading device and an information processing apparatus having the illuminating device, and more particularly an information processing apparatus (such as a copying machine, a facsimile apparatus, a scanner or an electronic blackboard), an image reading device adapted for use in such an information processing apparatus, an illumination device adapted for use in such an image reading device, and a light guide adapted for use in such an illumination device.
2. Description of the Related Art
For illuminating the image reading device of the information processing apparatus such as the facsimile apparatus, electronic copying machine or the like, there has conventionally been employed a discharge tube such as a florescent lamp or an LED array consisting of an array of a plurality of LED's. Particularly in recent years, the LED arrays are being used more widely, because compact and inexpensive products are requested for home-use equipment such as the facsimile apparatus.
An example of the illumination device utilizing such LED array will be explained with reference to FIGS. 1A and 1B, wherein shown are an LED array 41, a plane 42 to be illuminated, such as the surface of an original document, and LED chips 43. FIG. 1A shows the schematic structure of the illumination device employing an LED array, together with the original to be illuminated, while FIG. 1B shows an example of the illumination intensity distribution of the surface of the original when it is illuminated with the illumination device shown in FIG. 1A. As shown in FIG. 1B, a substantially uniform and high illumination intensity can be obtained by increasing the number of the LED chips, namely by densely arranging the LED chips. However, because of the increased number of the LED chips, it is difficult to achieve a sufficiently low cost, and to reduce the power consumption beyond a certain limit even though the power required for an individual LED is quite low.
A reduced number of the LED chips, or a less dense arrangement of the LED chips, for the purpose of cost reduction, will result in an uneven illumination intensity distribution on the illuminated surface, due to the increased gap between the LED chips, as will be explained in the following with reference to FIGS. 2A and 2B, wherein the same components as those in FIGS. 1A and 1B are represented by the same numbers.
FIG. 2A shows the schematic structure of the illumination device utilizing an LED array, together with the illuminated original, as in FIG. 1A, while FIG. 2B shows an example of the illumination intensity distribution when the original is illuminated with the illumination device shown in FIG. 2A. If the number of LED's in the array is decreased, there results, as shown in FIG. 2B, an extremely uneven illumination state in which the illumination intensity on the original surface is high in positions corresponding to the LED chips but is low in positions corresponding to the gaps between the LED chips. The precise original reading becomes difficult under such an illumination intensity distribution, and a circuit is required to compensate for the unevenness in the illumination intensity distribution, eventually leading to a higher cost.
FIG. 3 is a schematic perspective view showing the details of a linear light source similar to that explained above.
As shown in FIG. 3, such a linear light source is composed of LED chips 43, individually constituting a point light source, mounted linearly on a substrate 45 bearing electric wirings 49, and a voltage is applied between input terminals 48 of the wirings 49 to cause light emission from the LED chips 43, thereby constituting a linear light source.
FIG. 4 shows an elevation view of the light source, seen from a direction C shown in FIG. 3, and the light amount distribution on an illuminated surface (not shown), schematically illustrating the variation of the light amount corresponding to the positions of the LED chips 43. A curve 44 indicating the distribution of the light amount becomes higher in positions directly above the LED chips 43 but lower in positions corresponding to the gaps between the LED chips 43, because of the linear arrangement thereof. As a result, there is formed unevenness in the light amount corresponding to the arrangement of the LED chips 43. In reading image information with such a linear light source, the reflected light from the illuminated surface also involves unevenness in the light amount similar to that shown in FIG. 4, so that a large burden is required in the post-process such as image processing for improving the tonal rendition.
On the other hand, there is conceived a linear light source of the configuration as shown in FIG. 5, in which a light bulb, such as a tungsten lamp or a halogen lamp, is employed as the light source and the light emitted from the light source is developed into a linear form. In FIG. 5 there are shown an electric light bulb 1, such as a halogen lamp; a mirror 2 of a light condensing form, such as spherical or elliptical form; a translucent member 3 with a circular cross section, such as a quartz rod; an entrance face 4 where the light beam emitted from the light bulb 1 enters the translucent member 3; an area 5 for taking out the light beam, propagating in the translucent member 3, from the member by reflection or scattering, the area 5 being formed on a part of the translucent member 3 by forming a coarse surface or coating the surface thereof with light diffusing/reflecting paint; and a reflective face 6 provided at an end of the translucent member 3 opposite to the bulb 1 and formed either by evaporating a metal such as aluminum or applying light diffusing/reflecting paint on the end face of the translucent member 3 itself, or as a separate member. The translucent member 3 may also have a square or rectangular cross section.
The light beam L, emitted from the light bulb 1 and entering the translucent member 3 through the entrance face 4 thereof propagates in the member 3 by repeated reflections on the internal walls thereof, then is reflected by the end face opposite to the entrance face 4, and propagates again in the interior of the translucent member 3. In the course of repeated reflections, upon entering the above-mentioned area 5, the light beam is scattered therein and a part 1 of the light beam is released to the exterior through an exit face opposite to the area 5. The remaining part 12 of the diffused light beam, entering the exit face diagonally, is totally reflected thereon and propagates in the translucent member. The light reaching the entrance face 4 after repeated propagations is released therethrough to the exterior.
When the light bulb 1 is used as the light source, as the amount of light emission can be increased by the use of a larger electric power, there can be obtained a considerably high illumination intensity despite the light loss by the light emission to the exterior through the entrance face 4.
However, the use of the light bulb is associated with the drawbacks of a large electric power consumption in return for a high illumination intensity, difficulty in compactization of the device because of the large heat generation, and lack of maintenance-free character as in the case of LED's, since the electric light bulb has a service life considerably shorter than even that of the fluorescent lamp and has to be replaced when the light amount becomes low or when the filament is broken.
Consequently, the illumination device to be employed as the image reading light source for an information processing apparatus, such as a facsimile apparatus, preferably employs LED's as the light source and is adapted to emit the light beam from the LED's in a linear form. As another example of the illumination device employing the LED chips as the light source, there has been conceived a configuration shown in FIGS. 6A and 6B, which respectively are a schematic view of the illumination device together with an original to be illuminated, and a chart showing an example of the illumination intensity distribution on the illuminated surface 42 when the original is illuminated with the device shown in FIG. 6A. More specifically, the illumination device shown in FIG. 6A is similar to that shown in FIG. 5 except that the light source is replaced by an LED light source 71. In FIG. 6A, the components equivalent to those in FIG. 5 are represented by the same numbers.
The LED light source is available in various types, among which there is known so-called surface mounting LED chips convenient for compactization and actual mounting. FIG. 7 illustrates such a surface mounting LED light source, wherein shown are an LED chip 81; a substrate 82; a reflecting frame 83; translucent resin 84; and electrodes 85, 86 formed on the substrate 82. Such an LED light source is already available in a compact form, with the size of the light source itself of 2-3 mm and the height of 2 mm or less. As the electrodes 85, 86 are extended to the rear side of the substrate 82 through the lateral faces thereof, the light source can be efficiently mounted on the mounting substrate, merely by placing on the mounting substrate printed with cream solder and heating in a reflow oven. Consequently, the use of such an LED light source is more desirable for constructing a linear light source.
However, since such an LED light source has a directionality in the light emission as shown in FIG. 7, in illuminating the original in combination with the translucent member 3 as shown in FIG. 6A, an unevenness will result in the illumination intensity distribution, which is higher at the side of the LED light source 71 and is lower in the remaining part, as shown in FIG. 6B.
This is because the lights diagonally emitted from the LED light source 71 directly enter the area 5 of the translucent member 3, are scattered in the area 5 and released from the translucent member 3.
FIG. 8 is a schematic perspective view of another example of the conventional linear light source, in which light sources are provided on both ends of an oblong translucent member constituting a light guide 3. In FIG. 8, the light is emitted in a direction 11. The oblong translucent member 3 has a constant cross section, and the faces thereof are formed as mirrors except for the light-emitting face. The light is introduced from LED chips 71 provided on substrates 45 into the oblong translucent member 3 through the end faces thereof, and is released to the exterior either directly or after reflection by the mirror faces of the translucent member 3. FIG. 9 shows an elevation view, seen from a direction D shown in FIG. 8, and the illumination intensity distribution on the illuminated surface (not shown). As shown in FIG. 9, there is obtained a uniform light amount within an area a-c, but the level of light amount is low and is considerably different from that in the vicinity of the light source 10a, 10b and 10c are cross sections at the positions a, b and c of the oblong translucent member 3, and 44a, 44b and 44c indicate the illumination intensity distributions at the corresponding positions. Also hatched portions represent mirror faces (except for the light emitting face and light entering faces of the oblong translucent member 3).